The Nexus 6 is supported by the installer app starting with version 2.5.11.

The Nexus 6 (at least the 32GB version) has a completely filled system partition, even when file managers report some megabytes. The rSAP installer will abort with Bluetooth driver installation failed.

The easiest way to fix this is to remove an unused preinstalled app from /system/app. Titanium Backup seems to have a feature to do that or simply delete the app's folder with a root capable file manager.

Remember that any change to the system partition may trip future OTAs, because they check the system partition.